Ballast tube



i Patented Oct. 16, 1923.

UNITED. STATES PATENT OFFICE.

PAUL 'JIHOItNIE} WEEKS, OF CALDWELL, NEW JERSEY, A SSIGNOR TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

BALLAST TUBE.

' Application filed July 2,

To all whom it may concern: Be it known that I, PAUL THORNE WEEKS, a citizen of the United States, and a resident of Caldwell, in the county of Essex and State of New Jersey, have invented a new and useful Improvement in Ballast Tubes, of which the following is a specification. This invention relates to electrical resistance devices, and it has special relation to ballast tubes employed in connection with electrical apparatus requiring a nearly constant current of a predetermined value.

An object of the invention is to provide a ballast tube which will operate more expeditiously and more effectively than those heretofore used.

A more particular object is to provide a steadying resistance device with a rare gas which possesses the characteristic of being inappreciably absorbed 'by the resistance material employed therewith.

A further object is to provide a ballastresistance tube having a gaseous atmosphere of helium. Other objects will be apparent from the following detailed description. I

In the operation of audion tubes, incandescent electric lamps and other apparatus, it is desirable to maintain a nearly constant 0 currrent in order to secure a constant temperature. It is frequently the case that the potential of the circuit in which a device of this kind is to be connected is subject to variations either gradual or rapid,such that I if the device is connected in the circuit directly, or with a series resistance of the usual type, the desired degree of constancy of current is not obtained.

It has been customary, in order to provide against such variations in current occurring v within the device being operated to main tain a form of. resistance, known as a ballast resistance in circuit therewith. These resistances, depending on the particular material employed and the dimension thereof,

exercise corrective effects in different degrees and vary'as'to the range of temperature within which a small percentage increase of current produces a relatively large increase in resistance, and, correspondingly,

in voltage drop across such resistance. This range oftemperature 1s known as the critical range of each material. The relation 1921. Serial No. 482,063.

between the current and the voltage drop within this region is known as the current- 5 voltage characteristic.

Therefore, if a resistance having the proper critical range. and current-voltage characteristic be placed in circuit with an audion tube, for instance, which is connected to an ordinary commercial battery, any variations in voltages occurring in the battery potential will be balanced by the changing resistance and voltage drop in the balancing resistance and, therefore, the current in the audion tube will remain unaffected and be maintained at substanitally the desired value.

A ballast tube which has met with considerable commercial favor because of its 7 being fairly dependable to exercise the required corrective effects, is one employing a substantially pure-iron filament encased in an envelope of glass, or other desirable material, within which an atmosphere of hydrogen gas is placed to prevent oxidation of the iron and also to aid in securing the. desired current-voltage characteristic.

In ballast tubes of the character indicated, the cooling effect of a given gas varies with the density or pressure of the gas, or, in other words, the current required to heat the filament to a given temperature is dependent on the densityof the gas. As it is essential to the successful operation of the device that the filament be at a given temperature in order to secure the desirable current-voltage characteristic, it is necessary that the density be such that the desirable current will produce this temperature. The density of the hydrogen gas in a ballast tube employing an iron resistance may fall within that range of densities within ,which a slight change of density makes necessary a rela-- tively large change of current in order to maintain the temperature constant. Hence, any variation of density of the gas in the tube results in a corresponding shift inthe current-voltage characteristic so that the ballast resistance fails to maintain the current at the predetermined value.

In some cases, ballast tubes employing hydroge'n gas have been found to be subject to more or less gradual changes in the current voltage characteristic, or the mean value of current about which the tubes should exercise their corrective effects. The direction and amount of these changes appear to be dependent on the conditions under which the tubes have been previously operated. For instance, it has been fairly definitely established that, after a ballast tube has been operated at a high temperature for some time, the current-voltage characteristic will have shifted, so that the corrective effects exerted by the resistance in the ballast tubes will not take place at the given current value but at a higher current value. Later, however, if the tube is not used for some time or is operated at a low temperature, the current-voltage characteristic will shift back to approximately the original value.

The apparent explanation of this shifting phase of the current-voltage characteristic is that the hydrogen is probably absorbed by the material composing the filament, at certain temperatures, while, at other temperatures, it may be evolved. The absorption may be due to a chemical or aphysical action or a combination of both. Obviously, when the absorption takes place, a corresponding change in the pressure and density of the gas likewise occur, depending on the extent of such absorption, As pointed out previously, when the density changes but slightly, a marked changein current may take place, particularly, when the density is within the critical range; hence, when the density of the gas within the iron-hydrogen ballast tube is within this critical range and variations occur, due tochanging density conditions from absorption or evolution of the gas, the temperature of the filament will vary accordingly. This results in a change in the current value at which the tube will exercise its corrective effects.

This invention concerns itself with the employment of a gas other than hydrogen which will be more reliable in maintaining the current at the requisite value and which will not be absorbed to such an appreciable extent by the material composing the filament, thus providing against any marked change taking place in the current-voltage characteristic. At the same time, the cooling effect on the filament secured by the employment of hydrogen need not be sacrificed, as it is possible to employ a gas having this additional property.

In the accompanying drawings, Fig. 1

illustrates, conventionally, a ballast tube in which the invention is embodied; and

Fig. 2 is a diagrammatic illustration of the variation of the current-voltage characteristic that may arise from the employment of hydrogen gas.

Referring to Fig. 1, a resistance or filament 3 of iron or any other suitable material is secured by welding or clam ing, to a. pair of lead wires 4 and 5. The ead wires are connected, in the customary manner, to the exterior of the tube where convenient c0ntact may be had with the conducting wires from a source of electrical energy. The filament may be supported centrally with respect to a gas-tight container or an envelope 6 of transparent glass or other material.

The nature of the material employed as the resistance depends on whether it possesses a large positive temperature co-efiicient of resistance at the particular current value it is desired to operate a device with which the resistance may be connected in series. For the purposes of this invention, iron, in a substantially pure state, or even steel serves admirably.

The resistance material is prevented from oxidation and rapid disintegration by filling the containers in which it may be located with a gas which is practically inert with respect to it. In the tubes heretobefore employed, hydrogen has been found fairly satisfactory for this purpose.

As pointed out heretofore, ballast tubes provided with a filament or resistance of iron and a gaseous atmosphere of hydrogen do not regulate the current in a device being controlled as effectively as is desirable in certain instances, In Fig. 2, the first curve illustrates the current-voltage characteristic of a resistance of iron in an atmosphere of hydrogen when the lamp is first used. whereas the second curve indicates the shift which occurs in the current-voltage characteristic of the same lamp after it has been burned for a short time at a high temperature. As will be noted from these curves, the critical temperature at which the considerable increase in resistance occurs is reached between the range in current value of from .61 of an ampere to .63, this being the permissible range of current in the device being controlled thereby. The second curve, however, shows that the marked increase in voltage drop does not take place within the per missible range but occurs at a range of current considerably in excess thereof, or from 65 of an ampere to .667 of an ampere. By reason of such shift of the current-voltage characteristic, it is obvious that the varia-, tions ofvoltage will not be regulated by the ballast tube at the desired current value, which results in the audion tube, etc., to which it may be connected in series, being subject to these variations, with consequent interference with the successful operation of these devices. 1 i

In order to prevent a shifting of the current-voltage characteristic, it is necessary to provide an enclosed gas, the density of Which, remains substantially constant under I operating'conditions. As an incident to this requirement, the gas must be inert, physically and chemically, with respect to the material composing the resistance. Helium satisfactori y meets this requirement when steel lament.

Referring again to the first curve indicated in Fig. 2, when a tube employing helium is placed in series with an audion tube, by way of example, the mean current value desired to be maintained in such device may equal, say .625 amperes, which is the current value productive of the temperature in the resistance at which the latter manifests its corrective effects. After a heliumfilled tube has been burned a short time at a high temperature, instead of the currentvoltage characteristic shifting to the pos1- tion indicated by the second curve, it will be found that it has remained practically constant. It is also desirable to employ a gas which will serve to dissipate the heat from the resistance material either by conduction or convection, or both. Other rare gases possess these properties to a considerable degree but helium, as compare'd with hydrogen, is equally satisfactory.

Many other gases and materials may suggest themselves as being satisfactory for the purpose set forth and to produce the desired results, and such as may be considered to fall within the scope of the appended claims are contemplated b the present invention.

What is claime is: 1. A ballast tube com rising a resistance material having a rapi 1y increasing temperature coeflicient at a predetermined temperature and inclosed within an envelope containing a gas which is not appreciably absorbable by the resistance material.

2. A ballast tube comprising a filament having a rapidly increasin temperature coeflicient at a predetermine temperature, an

air-tight chamber inclosing the same and a S'Wltl'llll said chamber not a preciably absorbable by said filament an having high thermal conductivity.

3. A ballast tube comprising an iron filament, an air-tight chamber inclosing the same and a gas within said chamber not appreciably absorbable by said filament.

4. A ballast tube comprising an iron filament, an air-tight chamber inclosing the same and a gas within said chamber which said filament will not appreciably absorb,

said gas having the property of rapidly dissipating heat from said filament.

5. A ballast tube comprising a filament having a rapidly increasing temperature c0- efiicient at a predetermined temperature, an air-tight chamber inclosing the same and a rare gas within said chamber which said filament is incapable of appreciably absorbing.

6. A ballast tube comprising a filament having a rapidly increasing temperature coefiicient at a predetermined temperature, an air-tight chamber inclosing the same and a rare gas within said chamber with respect .to which said filament is substantially inert.

7. A ballast tube comprising a filament having'a rapidly increasing temperature coefiicient at a predetermined temperature, an air-tight chamber inclosin the same and a rare gas within said cham er said filament and said gas being practically physically and chemically inert with respect to each other.

8. A ballast tube comprising a filament having a rapidly increasing temperature 00- efiicient at a predetermined temperature, an air-tight chamber inclosing the same and .a rare gas within-said chamber with respect to which said filament is substantially inert, said gas having a high thermal conductivity.

9. ment and a hermetically-sealed envelope surrounding said filament and containing an atmosphere of {helium gas.

10. A ballast tube comprising a filament having a rapidly increasin temperature c0- efiicient at a predetermine temperature and a gas-tight envelope enclosing said filament and containing helium.

y 11. A ballast tube comprising a gas-ti ht envelope, a pair of leadin -in wires sea ed therein, a-base for said tu e, a filament in circuit with said leading-in wires, said filament having a rapidly increasing temperature coeflicient at a predetermined tempera ture and a gas within said tube not appreciably absorbable by said filament.

n testimony whereof, I have hereunto subscribed my name this 1st day of July, 1921. Y

' PAUL THORNE WEEKS.

A ballast tube comprising an iron fila- 

